About Kotsu

Kotsu is a free app for learning Japanese from zero. The name comes from コツコツ, “steadily, little by little”.

Privacy

Without an account, your progress stays on this device, in your browser’s storage. Nothing is sent anywhere unless you make a move link or turn on reminders.
A move link uploads a copy of your progress. The server keeps it for at most 24 hours and deletes it as soon as the link is used. The link itself is never stored, only a scrambled form of it that cannot be turned back into the link.
A backup file is saved only where you put it. Kotsu keeps no copy.
With an account, Kotsu keeps your Google account’s id, name and email and your study data, to sync your devices. It asks Google for nothing else and does not keep Google’s access tokens. “Delete my account” in settings removes the account and everything saved with it.
If you turn on reminders, Kotsu keeps this browser’s push address, your time zone and the last day you studied, with no account attached. Turning reminders off deletes them, and so does a month without opening the app.
To stop abuse, the server counts requests per network address for up to an hour. It stores a scrambled form of the address that cannot be turned back into it, and a daily clean-up deletes old counts.
Cookies: only the sign-in cookie, and only once you sign in. The light or dark theme you pick is kept in your browser.
Spoken Japanese uses your device’s own voices where it has them. Some browsers offer only online voices, and those send the words to the browser maker to be spoken.
Kotsu runs on Vercel (the app) and Neon (the database). No analytics, ads or trackers.

Sources and licences

The dictionary, kanji, sentence and stroke data come from these open projects. Kotsu’s converted copies are shared under the same licences.

JMdict
Words, readings, meanings and parts of speech.
© Electronic Dictionary Research and Development Group. CC BY-SA 4.0
KANJIDIC2
Kanji readings, meanings, stroke counts, grades and radicals.
© Electronic Dictionary Research and Development Group. CC BY-SA 4.0
JLPT Resources (tanos.co.uk)
JLPT levels of kanji and words (unofficial lists), converted by Bluskyo/JLPT_Vocabulary.
© Jonathan Waller. CC BY

Exact versions: ATTRIBUTION.md. JLPT levels are unofficial: the JLPT has not published word or kanji lists since 2010.